Many applications implemented in integrated circuits (ICs) require filtering of analog signals. Stable filters that can be implemented using standard CMOS processing technology without requiring accurate matching of analog components are highly desirable.
A common active analog filter used in integrated circuits is the switched-capacitor IC filter. The switched-capacitor IC filter rapidly charges and discharges a capacitor to create a resistance equivalent to that used in a passive filter. The filter can thus be tuned by changing the switch clocking frequency. Although they alleviate the problems of producing high value resistors on a semiconductor wafer, these filters are generally noisy. Additionally, although these filters provide flexibility in changing the effective resistance value in accordance with adjusting the switching rate, these switched-capacitor filters must still be accurately matched (e.g. impedance) with other analog components.
It is highly desirable to use digital filtering of analog input signals to avoid the matching problems of analog filters. Furthermore, it is easier to implement programmable filters in the digital domain than in the analog domain. Digital filters that use feed forward paths from the input signal are stable. However, digital filters with feedback loops can be unstable.
It is highly desirable to have a stable filter for analog signals that incorporate feedback while alleviating the matching requirements of associated with analog filters.